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#AskEnergySaver: Building Envelopes

May 30, 2014 - 3:16pm

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You're home's building envelope protects your home's interior from the outdoor environment to keep you comfortable all year long. But it can also contribute to higher heating and cooling costs. Learn how to improve its efficiency and save you money. | Photo courtesy of Kenneth Kelly, National Renewable Energy Lab.

What is a building envelope?

Your home's building envelope separates your home's interior from the outdoor environment to keep you comfortable and includes everything from its windows and doors to its walls, roof and foundation.

But if it has inefficient technologies, poor insulation and air leakage, your home's building envelope can also contribute to higher heating and costing costs -- which account for 43 percent of the average home’s energy consumption.

To help you save money by saving energy, we launched #AskEnergySaver -- an online series that gives you access to some of the Energy Department’s home energy efficiency experts. During 2014, experts from the Department and our National Labs will be answering your energy-saving questions and sharing their advice on ways to improve your home’s comfort.

Your home’s building envelope (its windows, doors, walls, roof and foundation) separates your home’s interior from the outdoor environment to keep you comfortable all year long. But if it includes inefficient technologies and poor insulation, your home’s building envelope can also contribute to higher heating and cooling costs -- which account for 43 percent of the average home’s energy consumption.

This month, we asked you to share your building envelope questions. To answer them, we turned to two scientists at the Energy Department’s Berkeley National Lab. With more than 20 of years experience in residential efficiency, Iain Walker’s research focuses on how ventilation and air leakage impact a home’s energy use. An expert in window technologies, façade systems and daylighting, Stephen Selkowitz heads Berkeley Lab’s Windows and Envelope Materials Group, where he explores ways to improve the energy efficiency of windows.

#AskEnergySaver are there any incentives for landlords to improve their rental's envelopes? So many would benefit-- from @Water_n_Science on Twitter

Iain Walker: Generally, no. Landlords can take advantage of the usual tax incentives and utility rebate programs if they want to invest in upgrading their property. But they usually don’t have a financial incentive to make energy efficiency upgrades because renters typically pay the utility bills.

I am getting ready to replace my leaking double pane casement windows with triple pane. Do you have any installation details for flashing and for insulating to minimize water and moisture intrusion and energy loss? What are the most energy-efficient window types in cold environments?-- from Daniel Katzenberger on Facebook

Stephen Selkowitz:Proper installation is critical to ensure a long life and optimal performance of any window. Because the specifics vary with the type of window being installed and the wall construction details, it’s best to refer to the manufacturers instructions or guides from industry associations like the American Architectural Manufacturers Association or the Window and Door Manufacturers Association.

Installation details should minimize air leakage. They should also be designed and installed to prevent water intrusion and to provide a pathway to the outside for any water that finds its way in.

As for what are the most energy-efficient window types in cold environments, you’ll achieve the biggest improvements in thermal performance when you switch from clear double glazing to double low-emissity (low-e) or triple glazing with low-e. There are smaller differences in properties of windows based on how they open (this is referred to as operator type by industry), but those decisions are more commonly based on the access or egress requirements, ventilation area and style. Look for NFRC performance labels -- administered by the National Fenestration Rating Council -- on windows for an apples-to-apples comparison of thermal properties that includes both glass options and operator type.

Is it true that triple-pane windows have a way longer payback time as compared to double-pane windows with low-e coatings?-- from Joseph Glynn on Facebook

SS: The added cost of going from double-pane windows with low-e to triple-pane windows should not be significant when you are buying from a manufacturer that offers a broad line of window sizes including many triple-pane window options. If the triple-pane glazed version becomes a "special order," then costs often get much higher.

The added value in energy savings and comfort will vary with location and climate. Triple-pane windows will have the best payback in the harshest (hottest/coldest) climates. Also keep in mind that the improved comfort from high performance windows can be a selling feature that adds value to a house just like granite counters and crown molding -- not all purchase decisions need to be framed in the context of "payback."

I live in Phoenix, Arizona, in a block home with shingle roofing. I am in the process of replacing my roof and thinking of putting a radiant barrier between the roofing materials. Are there other things I can do to help reduce my energy costs, like put more insulation in the attic? -- from Melissa via email

IW: First, a radiant barrier, which consists of a highly reflective material that reflects radiant heat rather than absorbing it, needs to face an air space. You should not put it between roofing materials -- either hang it below the roof deck or put it on the attic floor above the insulation.

Adding more insulation is a good investment, and at a certain point (probably above R40 or so) the insulation will be high enough that the addition of a radiant barrier will not make much difference to the load on your home. If your ducts are in the attic above the insulation, it is a good idea to make sure they are well sealed and insulated. The radiant barrier will reduce your duct losses and save you energy. Remember, any time you are dealing with insulation, what matters most is the installation -- so go with a good contractor.